In general an RFID tag system allows for objects to be labeled with tags such that when the tag is passed through the electromagnetic field of a reader/interrogator the object can be identified by reading the tag that is attached to the object. In use, RFID tags are attached in a wide variety of methods including being bolted to the item or simply glued to the inside of existing packaging or labeling. They can be encoded with a user-defined data at time of use, or pre-coded at time of tag manufacture numbering system or even a combination of both.
Radio frequency identification systems provide a number of advantages over paper and ink labels, such as bar code systems in that: a much greater degree of automation is permitted; clear line of sight is not required, tags can be obscured by dirt, paper, even other objects or packaging; reading distances can be greater; tags can be hidden either to protect the tag from damage in use or for security reasons; and in the case of read/write tags incremental information can be stored on the tags such as PO#, expiry date, destination, confirmation of an applied process, etc.
Those are just some of the advantages of RFID tags. The tag may be a single integrated circuit chip bonded to a flat, printed antenna, or could be a complex circuit including battery and sensors for temperature, position, orientation or any other required feature.
Specifically there are a great deal of different tag types that can be characterized as having one or more, but not limited to the following properties: passive, having no battery and therefore receiving all of its power required for operation from an electromagnetic field transmitted by the reader/interrogator or active using a self contained battery on the tag; collision arbitration, meaning that more than one tag can be read in the field of a single reader/interrogator at one time or non collision, meaning that only one tag can be in the field of the reader/interrogator at a time in order to insure a good read; multiple frequency where the data from the tag is carried on a different frequency from the data to the tag or single frequency where the carrier in both directions is the same; full duplex, where the tag is transmitting data back to the reader/interrogator while the reader/interrogator's transmitter is active or half duplex where the tag waits for the reader/interrogator's transmitter to go inactive before replying; solicited, where the tag must be commanded by the reader/interrogator before it transmits the data back, or unsolicited, where the tag transmits back as soon as it is powered up; active transmitter, where the tag has its own oscillator and transmitter or back-scatter, where the tag modulates the field set up by the reader/interrogator's transmitter; read only tag, which can be equated to an electronic barcode or read/write tag, which allows for the equivalent of a scratch pad on the tag. In either case tags can have different sizes of data that is transferred, different sizes of write-able memory, different accessing schemes to the data and different methods of writing; carrier frequency, is a function of the application, the physics of the objects being tagged, the range required and the radio frequency spectrum regulations of the country in which it is operating; data rate, is a function of the carrier frequency, the application needs and the radio frequency spectrum regulations of the country in which it is operating; data encoding methods can vary significantly however some form which encodes the data with the clock, such as Manchester encoding is generally used; packet protocol for data transmission from and to the tag has to be defined in terms of headers, addressing, data field types and sizes, commands, functions, handshaking, etc. etc.; error correction or detection codes, can be used by the tags to improve reliability of the tag data transfer, generally a CRC error detection only scheme is used, however the particular CRC code must be specified; additional signaling devices such as beepers or LEDs can be added to the tag to alert and direct the operator to a particular tagged object in the field; additional sensors, such as, for example temperature, can be added to the tag to record extreme conditions that the tagged object has been passing through.
As can be seen from the list above, there is an extremely wide variety of tag types that may be used or required by an application making it very hard to have one reader/interrogator handle all tag types. Typically there would have to be a specific reader/interrogator matched to the specific properties of each type of tag being used in the application.
For example, a typical low cost passive tag system with unsolicited tag response, would be implemented as follows; the reader/interrogator would first activate the tag by generating an electromagnetic field of a given frequency. Such an electromagnetic field can be generated, for example, by applying an alternating electrical current at a given frequency to a coil for low frequency near field systems commonly called inductively coupled systems or to an RF antenna for far field higher frequency systems.
The tag includes an antenna, which could be a dipole for far field systems or a coil for inductive systems tuned to the frequency of the interrogator's generated electromagnetic field. The electrical current thus generated in the tag's antenna is used to power the tag. Data is generally sent to the tag by modulating this interrogator generated electromagnetic field which is commonly called the exciter or illuminating field. The tag can send data back to the interrogator either by transmitting with its own transmitter with a separate frequency and antenna from the illuminating field or by modulating the illuminating field by changing the loading of the tag's antenna in what is commonly called a back scatter system. In any case, either the new electromagnetic field from the tag or the disturbances in the interrogator's illuminating field caused by the tag's back scatter system is detected by the interrogator. The data from the tag is thus decoded, thereby enabling the tag and the item to which the tag is attached to be identified. In some cases written to, as in the case of read/write tags by modulating the interrogator's generated electromagnetic field. Typical information that might be stored on the tags would be: PO#; expiry date; destination; confirmation of an applied process, etc.
The advantages and disadvantages of using different properties for the tag depend so heavily on the type of application that at this point there is no clear winner type of tag that will totally dominate the field. For example, in some cases range is an advantage, in other cases range is a disadvantage. Objects with high moisture or water content are not suitable for tagging with high frequency tags. Applications requiring high data rates or many tags in the field at any one time are not suitable to low frequency tags. Cost of the tag in relationship to the object being tagged and or the re-usability of the tag is a very important constraint in selecting tag properties.
As can be seen even from the few examples shown above, any application will be a compromise of tag properties in order to meet the application's need. In order to maximize the performance and meet the cost goals, the type of tag must be selected to match the application. Even if a single carrier frequency can be selected for an application differences in the other properties of the tag could still necessitate different reader/interrogators for the different tag types. Given that this is the case and that any large application may have different performance goals and therefore tag types, it is extremely advantageous to have a reader/interrogator that is flexible and can read many tag types simultaneously. This might even be mandatory in applications where there are different reader/interrogator types operating at the same carrier frequency and thus interfering with each other. Such a universal reader/interrogator would also solve the other great hurdle in implementing RFID tag systems, and that is the fear of obsolescence and not being able to read the next type of tag that may be required in the application.
In some situations, it is possible for an end user of the radio frequency identification system to include multiple readers, so that different tags using different protocols can be read. However, this is inefficient and expensive, as multiple readers would not be required if a single common standard for tags were used. Furthermore, multiple readers are likely to interfere with each other, especially if they operate at common radio frequencies.
Prior art readers for radio frequency identification systems have been devised to address some of the above-mentioned problems. For example, International patent application No. PCT/US98/10136, filed by AVID Identification Systems, Inc., on 14 May 1998, and entitled READER FOR RFID SYSTEM discloses a reader for reading tags of different protocols in a radio frequency identification system. According to this system, the identification signal from the tag is sensed by the inductive coil of the reader as described above in that the voltage across the coil is modulated in accordance with the code sequence programmed into the tag. The signal received by the coil is sent to a central processing unit for processing and decoding, where the signal is first analyzed by measuring the pulse width of the signal. The central processing unit then selects a tag protocol that is most likely to be the correct protocol based on the pulse width that has been measured.
The AVID radio frequency identification system may suffer from a number of shortcomings. For example, while the radio frequency identification system provides for reading of tags using different protocols in the same frequency range, it does not permit tags operating at different frequencies to be read by the same reader as the inductive coil of the reader is not operable for all electromagnetic frequencies. The AVID system is essentially an inductive based arrangement operating at a single frequency. Furthermore, the AVID system does not accommodate all of the tag properties and characteristics described above. Because the AVID system measures a single pulse width, at worst the system can only infer data rate from the pulse width and at best the system can only select from a very small group of tag types where the tag type would only be suitable if it has a distinguishing header pulse width. In general, the AVID system is not suitable for multiple carrier frequencies.
In view of the foregoing, there still remains a reader for a radio frequency identification system that may be used with tags operating at different frequencies with different protocols.